Since the spring, thousands of people have gathered near Cannon Ball, N.D., to protest the construction of a an oil pipeline near the Standing Rock Sioux Reservation. The protests have occasionally resulted in violent clashes with law enforcement.

During one tense confrontation last month, an explosion badly damaged one demonstrator’s arm in what was among the most serious injuries of the clashes. This week, as heavy snow blanketed the plains, Gov. Jack Dalrymple of North Dakota ordered a mandatory evacuation of the area where the demonstrations are taking place, as more activists have vowed to join the protest.

Here is a look at how the battle over the 1,170-mile pipeline has become an environmental and cultural flash point, stirring passions across social media and drawing thousands of protesters to camp out in rural North Dakota.

What is the latest from North Dakota?

Native Americans from scores of tribes have been gathering outside Cannon Ball — a town in south-central North Dakota, near the South Dakota border — to protest the Dakota Access pipeline. Starting with members of the Standing Rock Sioux Tribe, the protest has since grown to hundreds of people (estimates vary), most of them from Native American tribes across the country.

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Pipes being stacked for the Dakota Access oil pipeline last year at a staging site in Worthing, S.D.CreditNati Harnik/Associated Press

Last month, hundreds clashed with law enforcement officials who used water to repel protesters in below-freezing weather, attracting more attention to the conflict. During that confrontation, Sophia Wilansky, 21, who grew up in the Bronx, suffered one of the most serious injuries of the movement, her arm badly damaged from an explosion whose origin remains in dispute. As many as 2,000 veterans plan to join the demonstrators next week to serve as “human shields” against what they describe as a “militarized police force.”

In all, hundreds of people have been arrested since the protests began to attract widespread attention late this summer.

What does each side want?

The Dakota Access pipeline is a $3.7 billion project that would carry 470,000 barrels of oil a day from the oil fields of western North Dakota to Illinois, where it would be linked with other pipelines. Energy Transfer says the pipeline will pump millions of dollars into local economies and create 8,000 to 12,000 construction jobs — though far fewer permanent jobs to maintain and monitor the pipeline.

Members of the Standing Rock Sioux Tribe see the pipeline as a major environmental and cultural threat. They say its route traverses ancestral lands — which are not part of the reservation — where their forebears hunted, fished and were buried. They say historical and cultural reviews of the land where the pipeline will be buried were inadequate. They also worry about catastrophic environmental damage if the pipeline were to break near where it crosses under the Missouri River.

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Omaka Nawicakinciji, 7, of the Oglala Lakota Nation in South Dakota, protested the Dakota Access pipeline with his mother, Heather Mendoza, in Washington on Wednesday.CreditAlex Wong/Getty Images

As of early December, the federal government was still not allowing the pipeline company to drill under the river to finish a critical section that would link the pipeline together. The Army Corps of Engineers said in mid-November that it wanted to hear more from the Standing Rock Sioux, citing the importance of the water to the tribe as well as a long, painful history of lands’ being taken from the tribes.

It is unclear whether the corps will ultimately allow or block the pipeline. It is also anyone’s guess when it will make a final decision.

Are others fighting the pipeline?

Yes. State and federal agencies have approved the pipeline, and some farmers and ranchers have welcomed the thousands of dollars in payments that came with signing agreements to allow it to cross their land. But others oppose the pipeline.

In Iowa, one of the four states that the pipeline would traverse, some farmers have gone to court to keep it off their land. They say that Iowa regulators were wrong to grant the pipeline company the power of eminent domain to force its way through their farms. Most landowners in the 346-mile path of the pipeline through Iowa, however, have signed easements allowing it to be built across their land.

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The Standing Rock Sioux tribe and others are protesting the Dakota Access pipeline, which they say threatens water supplies and sacred lands.CreditCreditDaniella Zalcman

How many pipelines cross the United States?

The United States has a web of 2.5 million miles of pipelines that carry products like oil and natural gas, pumping them to processing and treatment plants, power plants, homes and businesses. Most of the lines are buried, but some run above ground.

While a natural gas line to a newly built subdivision is not likely to generate national controversy, proposed major pipelines like the Keystone XL, the Dakota Access or the Sandpiper in northern Minnesota have generated huge opposition from environmental groups and people living in their paths.

But pipeline spills and ruptures occur regularly. Sometimes the leaks are small, and sometimes they are catastrophic gushers. In 2013, a Tesoro Logistics pipeline in North Dakota broke open and spilled 865,000 gallons of oil onto a farm. In 2010, an Enbridge Energy pipeline dumped more than 843,000 gallons of oil into the Kalamazoo River in Michigan, resulting in a cleanup that lasted years and cost more than a billion dollars, according to Inside Climate News.

In a 2012 examination of pipeline safety, ProPublica reported that more than half of the country’s pipelines were at least 50 years old. Critics cited aging pipelines and scant federal oversight as factors that put public health and the environment at risk.

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A version of this article appears in print on , on Page A12 of the New York edition with the headline: A North Dakota Oil Pipeline Battle: Who’s Fighting and Why. Order Reprints | Today’s Paper | Subscribe